Burning coal superclean

The Advanced Hybrid filter puts rows of bag filters between the collection plates, a first in the industry.

(Inset right) Electrostatic precipitators use electrodes to give particles an electric charge that draws them to collection plates.

A new clean-air system from the University of North Dakota, dubbed the Advanced Hybrid filter, eliminates more than 99.99% of the microscopic particulate matter released by burning coal. According to researchers, most coal-fired plants today capture almost that much particulate matter (99.95%). They compare the boost in efficiency to the difference between a car getting 20 mpg and one that's hitting 2,000 mpg. The new clean-air technology is already in action at a power plant in South Dakota.

Key to the filter system is a combination of two fly ash-capturing technologies: Electrostatic precipitators and Gore-Tex membrane filter bags. Electrostatic precipitators use electrodes to give particles an electric charge that draws them to collection plates. The Advanced Hybrid system puts rows of bag filters directly between the collection plates. The bags stop the particles that get by the collectors and, periodically, inject air in reverse through the filters, dislodging built-up particles and sending them back toward plates to be collected. This not only keeps the bags clean, but also uses less energy drawing air. Reportedly air coming out of the plant is cleaner than that going in, with respect to fine particulate.